Apparatus and method for top removal of granular material from a fluidized bed deposition reactor

ABSTRACT

Removal of the product from the top of the reactor enables a decreased disengaging height and provides a passive means of controlling the bed level despite deposition increasing the weight and height of the bed. The savings from reducing the disengaging height allow use of a taller fluidized bed in a shorter overall reactor length and thus provides increased production with reduced reactor cost. The separation of the gas inlet from the product outlet allows the gas inlet area to be cooler than the product outlet. The separation of the product grinding, caused by the inlet gas, from the product outlet reduces the loss of seed in the product and produces a more uniform product. Removing the hot product and the hot gas at the same place allows energy recovery from both in a single step.

CROSS REFERENCE TO RELATED APPLICATIONS

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STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

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DESCRIPTION OF ATTACHED APPENDIX

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BACKGROUND OF THE INVENTION

This invention relates generally to the field of deposition reactors andmore specifically to an apparatus and method for top removal of granularmaterial from a fluidized bed deposition reactor.

Fluidized bed reactors have a long tradition in the chemical industrywhere the bed usually consists of a finely divided valuable catalystwhich makes it necessary to design the reactors to prevent catalystlosses. Thus was developed the practice of requiring a large disengagingheight above the bed surface and of using cyclones to capture the finedust and return it to the bed. A concept called total disengagingheight, or TDH, was developed to estimate the height where all theparticles that would settle out by gravity had settled out. Internalcyclones were provided at this height to capture the finer dust andreturn it to the bed. Whenever the catalyst was removed it was removedfrom the bottom by gravity. Other reactors called dilute phase ortransport reactors entrained all the solids up through the reactor andout the top, but these reactors did not have a recognizable bed. Whenthese gas-solids reactor concepts were applied to the design ofdeposition reactors where gases are introduced to make the bed particlesgrow, the dilute phase reactor had the major problem that it producedmainly a fine dust which was undesirable. Thus the majority ofdeposition reactors have been fluidized beds and so the basic design offluidized beds with a large disengaging height and bottom solids outlethas been used. The idea of internal cyclones has seldom been usedbecause of the deposition on the outside of the cyclones and theproblems of reintroducing particles without plugging the cycloneoutlets. Since some fine dust is always made, most deposition reactorshave external cyclones or filters to trap the dust and prevent damage tothe equipment used to recover the effluent gases. Thus the historicapproach has been to have removal of the product from the bottom,provide a large disengaging height to minimize product loss, and useexternal dust removal. The primary use for deposition reactors is inhigh purity silicon deposition and Lord in U.S. Pat. No. 6,451,277 inFIG. 1 b describes a bed heating method which removes beads from nearthe top of the bed and then heats them and returns them to the bed. Itis notable that the product, 3, is still removed from the bottom. In theabove patent this bed heating method is rejected in favor of a preferredoption where the beads are removed by gravity from the bottom thenreheated and returned to the bed in a pulsed mode. Lord in U.S. Pat. No.6,827,786 provides a detailed description of a multistage depositionreactor which takes advantage of increased bed height to produceadditional silicon by use of additional gas injection points along theside of the reactor. In this design the seed generation by grinding isspread out along the reactor because of the extra nozzles and somedeposition occurs further from the inlet, but most of the grinding anddeposition occurs in the bottom where the solid product is removed. Lorddiscusses, Col3 line 25, the “De Beers” paper which showed the need forsome residence time and temperature to fully crystallize the product anddehydrogenate the beads. He does this in the pulsed bead heater at hightemperature and with short residence time. Lord and his many referencesdo not discuss energy recovery from the effluent gas although Lord inU.S. Pat. No. 5,798,137 and 6,451,277 discusses the use of the heat fromthe outgoing product to heat the incoming gas.

The primary deficiency of the prior technology is staying with theinherited fluid bed design of a bottom outlet and large disengagingspace and accepting the inherent conflicting demands caused byintroducing the cold deposition gas, which also provides the bulk of theseed generation by grinding, at the same location as the hot product wasremoved. Lord in various patents attempts to deal with the heat and seedgeneration problem by spreading out the gas injection, but sufficientgas to fully fluidize the bed must be injected at the bottom, so thereis a limit to what can be accomplished in this manner. Inevitably thebottom temperature must be maintained above 800° C. to provide theneeded crystallization, and some seeds are lost to the product which isin turn contaminated with broken “seed beads.” The combination of hightemperature and high deposition gas concentration leads to rapidreactions, increased wall deposits and increased risk of agglomerationand plugging.

This multistage design approach also leads to tall reactors and thereare cost and manufacturability issues in producing the high purityliners for such reactors which restrict the number of stages and henceproduction capacity of a given diameter reactor. It is also necessary tomeasure the bed level and take corrective action by removing some of thebed as the bed grows by opening valves and changing purge flows to allowthe right amount of beads to leave the bed. Errors or stuck valves canlead to situations where the bed is too high or too low. Both of theseconditions are undesirable upsets.

BRIEF SUMMARY OF THE INVENTION

The primary object of the invention is to provide a shorter reactor withgreater production.

Another object of the invention is to provide a passive method of levelcontrol.

Another object of the invention is to provide a better quality product.

A further object of the invention is to reduce the need for hightemperature at the bottom of the reactor.

Yet another object of the invention is to reduce the risk of plugging.

Still yet another object of the invention is to reduce the thickness ofwall deposits.

Another object of the invention is to reduce the pressure in the productremoval system.

Another object of the invention is to recover energy.

Other objects and advantages of the present invention will becomeapparent from the following descriptions, taken in connection with theaccompanying drawings, wherein, by way of illustration and example, anembodiment of the present invention is disclosed.

In accordance with a preferred embodiment of the invention, there isdisclosed an apparatus and method for top removal of granular materialfrom a fluidized bed deposition reactor comprising: removal of theproduct from the top of the reactor together with the effluent gas,separation of the granular product from the effluent gas, simultaneousrecovery of heat from the product and the gas and optional further dustand heat recovery.

The technical benefits of this design are passive level control,decreased disengaging height, taller fluidized bed in a shorter reactor,separation of gas inlet from product outlet, separation of productgrinding from product outlet and energy recovery which in turn lead tolower capital and operating cost, a better quality product and greaterthroughput for a given reactor diameter.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings constitute a part of this specification and includeexemplary embodiments of the invention, which may be embodied in variousforms. It is to be understood that in some instances various aspects ofthe invention may be shown exaggerated or enlarged to facilitate anunderstanding of the invention.

FIG. 1 is a schematic diagram illustrating the operation of a fluidizedbed deposition reactor of the prior art with bottom removal and a largedisengaging space.

FIG. 2 a is the same diagram modified to show the benefits of theinvention.

FIG. 2 b is a detailed schematic of the top of the reactor showing thegranular particle removal mechanism.

FIG. 3 is a schematic of a product separator with integrated heatrecovery.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Detailed descriptions of the preferred embodiments are provided herein.It is to be understood, however, that the present invention may beembodied in various forms. Therefore, specific details disclosed hereinare not to be interpreted as limiting, but rather as a basis for theclaims and as a representative basis for teaching one skilled in the artto employ the present invention in virtually any appropriately detailedsystem, structure or manner.

Turning first to FIG. 1 there is shown a schematic of a typicalfluidized bed deposition reactor comprising a containment vessel orliner, 111, of a height, 144, a gas introduction means, 112, an optionalgas distribution means, 113, a bottom product removal means, 114, a bedheating means, 115, a gas/dust mixture exit, 116, a connecting means,127, a dust/gas separation means, 117, a dust removal means, 118, and agas exit, 119. The containment vessel, 111, surrounds a bed of granules,120, fluidized by gas bubbles, 121, and having an average top level,122, above which product granules, 123, thrown up above the bed describearcs as they rise from random impact within the bed then fall undergravity in a reduced disengaging space, 124, while the small entraineddust particles, 125, continue up and leave with the effluent gas, 126,through the gas/dust mixture exit, 116, through the connecting means,127, then enter the dust/gas separation means, 117, where most of thedust, 125, is removed from the gas, 126, and then ultimately leaves thesystem via the dust removal means, 118, while the gas, 126, and residualdust leaves via an exit, 119. The differential pressure meter, 128,measures the difference in pressure between the bottom product removalmeans, 114, and the gas exit, 119. This measurement indicates the level,122, of the bed of granules, 120. The bottom removal means, 114, is usedto control the top level, 122, to maintain the disengaging space, 124,so that the product granules, 123, are returned to the bed of granules,120, and are thus removed by the bottom product removal means, 114. Thisis a very general schematic and the patent literature is full of thevarious methods and machines that have been proposed to fulfill theserequirements. It is possible to have more than one gas entry and toavoid the gas distribution mechanism; the heating means can be of manydifferent kinds, and the dust removal can be done by a cyclone as shown,by a filter or by another gas cleaning device.

In accordance with the present invention, FIG. 2 a shows a schematicsimilar to FIG. 1 but modified to remove the granular product from thetop via a gas/granular separator means, 230, inserted before theeffluent gas enters the gas/dust separation means, 217. A furthermodification is the removal of the differential pressure transmitter,128, shown in FIG. 1, which is not required for bed level control. Theinvention thus comprises a containment vessel or liner, 211, of aheight, 244, a gas introduction means, 212, an optional gas distributionmeans, 213, an optional bottom product removal means, 214, a bed heatingmeans, 215, a gas/dust/granular mixture exit, 216, a first connectingmeans, 241, a gas/granular separator means, 230, with a granular removalmeans, 231, an optional heat recovery means, 242, a further connectingmeans, 229, a gas/dust separation means, 217, a further optional heatrecovery means, 243, a dust removal means, 218, and a gas exit, 219. Thecontainment vessel, 211, surrounds a bed of granules, 220, fluidized bygas bubbles, 221, and slugs, 240, and having an average top level, 222,above which some granules, 223, thrown up above the bed describe arcs asthey rise from random impact within the bed then fall under gravity in areduced disengaging space, 224, while some granules, 236, and the smallentrained dust particles, 225, continue up and leave with the effluentgas, 233, through the gas/dust/granular mixture exit, 216, theconnecting means, 241, and into the gas/granular separator means, 230,where the granules are removed via the granular removal means, 231. Theremaining gas and dust leave through the gas/dust top exit tube, 229,then enter the gas/dust separation means, 217, where most of the dust,225, is removed from the gas, 233, and ultimately leaves the system viathe dust removal means, 218, while the gas, 233, and residual dustleaves via an exit, 219.

To accomplish the removal of large granules the average top level, 222,is very close to the gas/dust/granular mixture exit, 216, andconsequently some of the product granules, 236, thrown up above the beddo not describe arcs as they rise then fall under gravity in thedisengaging space, 224, but continue with the entrained dust, 225, outthe gas/dust/granular mixture exit, 216. Since the average bed level,222, is closer to the exit, 216, the bed level, 222, can be tallerand/or the overall height, 244, can be shorter compared to the prior artas shown in FIG. 1.

Turning to FIG. 2 b there is shown in detail the various mechanismswhich cause the product granules, 236, to be carried out the gas exit,216. The basic mechanism is the random ejection of product granules,236, from the top of the bed, 222, and the pneumatic conveying of thesegranules out the gas/dust/granular exit, 216. In addition the bed leveloscillates up and down due to the formation of gas slugs, 240, whichlift sections of the bed up to the high level, 232, until they breakthrough and the bed level recedes to the low level, 234. It is alsopossible for the bed to reach extra high levels, 235, where the bed isabove the exit briefly. The exit tube, 241, can be attached to the exit,216, at 90° as shown or sloped above or below the horizontal. The anglechosen can be determined by the application of standard pneumaticconveying calculations using the gas velocity in the exit tube, 241.

Turning now to FIG. 3 there is shown a more detailed schematic of aproduct separator, 330, with an integrated heat recovery system, 301,suitable for high temperature and high purity applications. Thegas/dust/granular mixture, 333, enters the product separator, 330,through an inlet, 357, which goes through the heat recovery system, 301,via a penetration, 358; the gas and dust, 356, then separate to the topand exit via the exit tube, 329, while the granules, 336, separate tothe bottom exit, 331, where it is fluidized by a purge stream, 359, andwithdrawn as needed.

The heat recovery system, 301, is comprised of a heat transfer fluid,360, contained in a container, 351, which is shaped to capture heat,350, from the wall of the product separator and has an inlet, 354, andan outlet, 355, for the heat transfer fluid, 360. The container can usevarious heat transfer fluids such as water or hot oil. It is usuallyadvantageous for the container to be a pressure vessel to permit heatrecovery at higher temperatures. The heat may be transferred from thewall to the container by radiation, conduction or convection andwell-known heat transfer techniques can be used to enhance the heattransfer from the gas and solids to the wall. Similarly, well-knowngas-solids removal techniques, such as cyclones or filters, can be usedto enhance the gas-solids separation.

In a particularly advantageous design, the heat is transferred byradiation from the hot surface of the product separator to a pressurizedcontainer which has water, 352, coming in through the inlet, 354, andsteam, 353, leaving through the exit, 355.

An example using FIG. 2 would be as follows. The diameter of thecontainer is 300 mm, the overall height of the liner, 244, is 7 meters,the average bed level, 222, is 6 meters, the high level is about 6.6meters and the low level is about 5.4 meters. The gas superficialvelocity at the top of the container is 4.7 ft/s (1.4 m/s). The averageparticle size of the granules is 1 mm and the terminal velocity is 21.8ft/s (6.56 m/s). The particle terminal velocity is thus about 4 timesthe superficial gas velocity. This means that in order to carry thegranules out of the reactor, the local velocity in areas just above thebed must have local surges where it is 4 times higher than average.Velocity surges of this magnitude occur close to the top of the bed atabout 20 cm above the bed. The slug, 240, has a maximum length of about1.2 meter, and so the periodic growth and bursting of the slug providesthe variation in height of 1.2 meters between low and high level. As theslug bursts, it also accelerates the granular particles which are. thenentrained out of the reactor. Thus the granular removal varies with thepulsing of the slugs, 240

In comparison, for FIG. 1 under similar operating conditions with anaverage bed level, 122, of 6 meters, the overall height would be 10meters in order to allow for the disengaging space normally requiredunder the prior art.

The granules and gas at the bottom of the reactor are at 700° C., thenare heated up and leave the reactor as stream, 233, via exit, 216, at atemperature of 800° C. They enter the cyclonic product separator, 230,through a tangential inlet which forces the gas and solids to the wallof the vessel to improve gas to wall heat transfer. The diameter of thecyclone is 10 inches (250 mm) and the length is 6 ft (1.8 m). This islonger than needed for solely the solids removal in order to providesufficient surface area for heat transfer. The gas and granules bothleave at 600° C. The dust/gas separator, 217, is of a similar size butonly removes about half the heat because of the reduction in thetemperature difference. The gas and dust then leave the dust/gasseparator at 500° C. Both heat recovery systems recover the heat as 150psig steam, which is a standard utility useful in the facility for avariety of purposes and thus always in demand.

While the invention has been described in connection with a preferredembodiment, it is not intended to limit the scope of the invention tothe particular form set forth, but on the contrary, it is intended tocover such alternatives, modifications, and equivalents as may beincluded within the spirit and scope of the invention as defined by theappended claims.

1. An apparatus and method of operation for top removal of granularmaterial from a fluidized bed deposition reactor comprising: a containerof a set height with at least one gas inlet at or near the bottom and atleast one gas and solids outlet at or near the top, a bed of granularparticles of variable height which is fluidized and deposited on by agas flow, a gas/granular product separator means and a method ofoperation where the height of the bed of granular particles is allowedto increase until it reaches a stable height.
 2. An apparatus forrecovering heat while separating the granular product comprising: one ormore product separator means and one or more heat recovery means.
 3. Anapparatus of claim 1 where at least one means for granular productremoval is also provided at the bottom.
 4. An apparatus of claim 2 whereat least one of the heat recovery means is primarily by radiation to aheat recovery boiler.
 5. An apparatus of claim 2 where more than oneseparator means are used to provide more than one product streams ofdifferent average particle size.